Optical spectroscopy techniques have been developed for a wide variety of uses within the medical community. For example, pulse oximetry and capnography instruments are in widespread use at hospitals, both in the surgery suites and the post-op ICU's. These technologies have historically been based on absorption-based spectroscopy techniques and have typically been used as trend monitors in critical care environments where it is necessary to quickly determine if a patient's vital parameters are undergoing large physiologic changes. Given this operating environment, it has been acceptable for these devices to have somewhat relaxed precision and accuracy requirements, given the clinical need for real-time point-of-care data for patients in critical care situations.
Both pulse oximeters and capnography instruments can be labeled as non-invasive in that neither require penetrating the outer skin or tissue to make a measurement, nor do they require a blood or serum sample from the patient to custom calibrate the instrument to each individual patient. These instruments typically have pre-selected global calibration coefficients that have been determined from clinical trial results over a large patient population, and the results represent statistical averages over such variables as patient age, sex, race, and the like.
There is, however, a growing desire within the medical community for non-invasive instruments for use in such areas as the emergency room, critical care ICU's, and trauma centers where fast and accurate data are needed for patients in potentially life threatening situations. One such measurement needed in these environments is the blood and/or tissue pH level, which is a measure of the free hydrogen ion concentration. This is an important measure of intracellular metabolism. Biological processes within the human body require a narrow range of pH for normal function, and significant changes of pH from this range may be life threatening.
In addition to pH, it is also typical for other physiologic parameters such as the blood gases (O2 & CO2), blood electrolytes, cardiac-event enzyme markers, and other blood chemistry parameters such as glucose, to be measured and monitored during critical care treatment. Technologies for making these measurements have been in place for nearly fifty years in hospital laboratories. These measurements are made from blood samples drawn from the patient which are then sent to a laboratory for analysis. These laboratory measurements are typically made with electrochemical sensors.
Recent developments in non-invasive optical technology hold the potential that some of these measurements may be made at the point of care with sufficient precision and accuracy to carry out critical care monitoring and treatment. Also, there has been an increased interest in utilizing both the absorbance and fluorescence properties of naturally occurring biological molecules as physiologic markers for non-invasive optical measurements. Both of these techniques are complicated by the patient-to-patient variability in skin texture and chemical composition, both of which affect the optical properties of the skin and make universal calibration of such devices difficult.